The current submission of the Program Project Grant (PPG), Cellular Decisions of Differentiation in the Gl Tract integrates the efforts of three investigators (two basic science and one clinical) from three Departments at the University of Michigan. The central goals of the proposed studies generally remain the same as the prior two cycles: (a) To understand how gastric epithelial cells develop and maintain their identity by expressing or responding to developmental signaling pathways activated by the peptide morphogen sonic hedgehog (Shh) or the transmembrane signaling receptor Notch (b) To investigate how the patterns of cellular differentiation in the gastric corpus, gastric antrum and intestine use these signaling pathways to maintain homeostasis or respond to environmental stress, e.g., chronic inflammation. Subproject #1 entitled Modulation of myeloid cell phenotype by hedgehog signals will expand upon the intial translational observation that the Hedgehog target gene Glil expressed in myeloid cells modulates the epithelial response to inflammation. Subproject #2 entitled Role of Hedgehog signaling in chronic gastritis and metaplasia will examine the role of chronic inflammation, specifically proinflammatory cytokines in mediating changes in patterns of cellular differentiation, e.g., metaplasia in the gastric corpus and hyperplasia in the antrum. Subproject #3 entitled Notch Regulation of Gastric Epithelial Cell Homeostasis and Tumorigenesis will explore the effect of the Notch pathway on Lgr5+ stem cells in the antrum and its possible interaction with the Hedgehog pathway. The PPG will support one service core (Cell Biology Core) to efficiently, process, analyze and coordinate tissue samples and flow cytometric analysis between the three projects. In summary, the PPG will use a variety of different mouse models and cell or molecular -based approaches to understand how gastric and intestinal cells maintain their homeostasis, but then modify their cellular patterns of differentiation in response to chronic inflammation ultimately directing the cll towards a pro-proliferative phenotype.